The main research activities supported in this period can be divided into basic and applied studies of the periodontal microbiota. In addition several peripheral projects were also completed. The basic studies were focused on taxonomic studies of newly classified organisms, one of which was isolated and fully characterized in our laboratory, namely Centipeda periodontii. Another project along these lines was conducted at the Forsyth Dental Center in Boston. We contributed to the morphologic characterization of Wolinella curvy, a newly named oral bacterium. The applied studies included completion of a 3-year study to compare the periodontal status of gingivitis patients on a standard and an experimental prophylactic regimen, the latter being based on microscopic monitoring of the gingival microbiota. The results indicated that (1) very few surfaces with gingivitis progressed to periodontitis, (2) most increases in the severity of gingivitis were due to relatively few subjects, and (3) that the above findings should be taken into consideration when other experiments of this type are designed. A 4-year longitudinal study was started to examine the comparative periodontal status of a population of patients with a history of treated periodontitis on maintenance regimens similar to those in the 3-year gingivitis study. Preliminary results indicated that relatively few subjects account for most episodes of disease recurrence. No difference could be detected between differential dark-field microscopic (DDFM) counts of selected bacterial morphotypes at sites of disease recurrence and control samples from non-affected sites in the same subjects. Possible reasons for this finding were discussed. The reliability of DDFM counts for diagnostic purposes was explored using previously acquired data to generate a receiver operating characteristic curve to serve as a basis for determining critical values for a diagnostic test. Proportions of spirochetes plus motile rods in the 15-20% range appear to be appropriate critical values for such a test.